<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        body {
            height: 2000px;
        }

        .box {
            width: 100px;
            height: 100px;
        }

        .first {
            right: 100px;
            background: yellow;
            position: fixed;
            top: 100px;
        }

        .secend {
            background: red;

        }

        .secend:before {
            display: block;
            content: 'b';
        }

        .secend:after {
            content: 'a';
            display: block;
        }

        .third {
            background: blue;

        }

        .test_fatche {
            position: absolute;
            top: 300px;
            left: 200px;
            width: 300px;
            height: 300px;
            border: 1px solid #000000;
        }

        .aaa {

        }
    </style>
</head>
<body>


<div class="box first"></div>
<div class="box secend">
    内容
</div>
<div></div>
<div class="box third">

    <div class="test_fatche">
        <div>
            <div class="box aaa" style="background: brown">

            </div>
        </div>
    </div>
</div>


</body>
</html>